1. Field of the Invention
The present invention relates to an apparatus for reading a color image by using a solid-state imaging device or the like, and more particularly to a color image reader which directs light from an object to a sensor, such as a solid-state imaging device, through a focusing optical system and color decomposition means.
2. Related Background Art
A prior art apparatus, which line-scans an object such as a document sheet in a sub-scan direction and color-reads an image thereof by a solid-state imaging device (such as CCD) array is shown in FIG. 1. In FIG. 1, an information light on a portion of a document sheet 1, illuminated by a light from an illumination light source (not shown), is decomposed and separated into three colors by a three-pieces (3p) prism 20 through an imaging optical system 19, and focused to and read by three one-line CCD sensors 21, 22 and 23.
In the prior art apparatus, three independent sensors are required. Further, since the 3p prism 20 requires high precision to manufacture, its cost is high. In addition, adjustment of the focusing light beam and the sensors 21, 22 and 23 is required for each of the three sensors and the difficulty in its manufacture is high.
It has been proposed to arrange three one-line sensors in parallel on one substrate with a predetermined spacing therebetween so that they functions as a monolithic 3-line sensor.
Such a 3-line sensor 24 is shown in FIG. 2A. Spacings S.sub.1 and S.sub.2 between the three lines 25, 26 and 27 are, for example, 0.1-0.2 mm by various manufacturing conditions. Widths W.sub.1 and W.sub.2 of a single element 28 as shown in FIG. 2B are, for example, 7 .mu.m.times.7 .mu.m and 10 .mu.m.times.10 .mu.m.
A known color image reader, which uses such a monolithic 3-line sensor as the photo-sensing device, is shown in FIG. 3. In FIG. 3, when information on a document sheet 1 is read by line-scan along a sub-scan direction, light from the document sheet 1 is focused through optical system 29 and decomposed and separated into three light beams of three colors by color decomposing beam splitters 30 and 31, having dichroic selective transmission films applied thereto, and are focused onto line-elements on the monolithic 3-line sensor 32.
As shown in FIG. 3, where a thickness of the beam splitters 30 and 31 is X, a line-to-line distance on the sensor 32 is 2.sqroot.2X. Thus, if the line-to-line distance (2.sqroot.X) is 0.1-0.2 mm as described above, the thickness (X) is 35-70 .mu.m. This thickness is not easy to manufacture when a required flatness of the plane is taken into consideration.
On the other hand, distances between the center line 26 of the monolithic 3-line sensor and two other lines 25 and 27 are usually equal and integer multiples of a pixel size in the sub-scan direction (W.sub.2 in FIG. 2B).
The reason is as follows. As seen from FIG. 4A. when the image is read by the monolithic 3-line sensor by using only a conventional focusing optical system 45, three positions 25', 26' and 27' (FIG. 4A) on the document sheet are simultaneously read by the three lines 25, 26 and 27. Since three color (R, G, B) signal components for one point are not simultaneously read, it is necessary to collate and compose them after reading.
Where the line-to-line distances S.sub.1 and S.sub.2, as shown in FIG. 4B, are integer multiples of the pixel size and a redundancy line memory is provided to delay the G and R signals, for example, relative to the B signal, the three-color composition is attained relatively easily. For this reason, the integer multiple distance is selected. However, since a redundant line memory is required for each line-to-line distance, a plurality of expensive line memories are required. This is disadvantageous from a cost standpoint, and an inexpensive color reader cannot be provided.
A color image reader which uses a blazed diffraction grating as a color decomposition means is disclosed in U.S. Pat. No. 4,277,138 (counterpart DE 2,645,075).
However, it pays no attention to the construction of a sensor which reads an image of an object.